Electric motor with plural stator components

ABSTRACT

An electric motor apparatus comprises a rotor and a stator formed of at least two stator components, each of the at least two stator components having a substantially hollow cylindrical form. The rotor is mounted within the at least two stator components on a rotational mounting such that the rotor can rotate about a longitudinal central axis with respect to the stator. Each of the at least two stator components has at least two protrusions arranged at different circumferential points on an inner surface of the at least two stator components. Each protrusion has a winding mounted thereon. Control circuitry generates control signals to control power supplied to the windings on each stator component such that power can be controlled to each stator component independently. The stator components are mounted adjacent to each other along the longitudinal central axis and rotationally offset with respect to each other such that the two protrusions on one of the stator components are offset with respect to the protrusions on an adjacently mounted stator component, such that a portion of each winding that extends beyond a longitudinal end of the protrusions on one of the stator components fits within a gap between windings mounted on the adjacently mounted stator component.

CROSS-REFERENCE

This application is a continuation of U.S. application Ser. No.14/105,201 filed Dec. 13, 2013, the entire contents of which areincorporated herein by reference in this application.

TECHNICAL FIELD

The technical field relates to electric motors.

BACKGROUND

There are several different ways of configuring electric motors each ofwhich has its own associated advantages and disadvantages. Many of theseconfigurations provide very efficient operations. However, this highefficiency is generally restricted to a particular optimum operatingfrequency of the motor and when the motor moves away from this operatingfrequency the efficiency may drop dramatically. In order to address thisproblem, motors of the prior art have been operated in conjunction withgearing systems, such that they can drive axles at different speeds,while still operating close to their own optimum rotation frequency. Theprovision of gears adds cost to the system and can themselves reduceefficiency.

It would be desirable to be able to increase the range of efficientoperation of an electric motor.

SUMMARY

Viewed from a first aspect the present invention provides an electricmotor apparatus comprising:

a rotor; and

a stator formed of at least two stator components, each of said at leasttwo stator components having a substantially hollow cylindrical form;

said rotor being mounted within said at least two stator components on arotational mounting such that said rotor can rotate about a longitudinalcentral axis with respect to said stator;

each of said at least two stator components comprising at least twoprotrusions arranged at different circumferential points on an innersurface of said at least two stator components, each of said at leasttwo protrusions having a winding mounted thereon; and

control circuitry configured to generate control signals to controlpower supplied to said windings on each of said at least two statorcomponents, such that power can be controlled to each of said at leasttwo stator components independently of each other; wherein

said at least two stator components are mounted adjacent to each otheralong said longitudinal central axis and rotationally offset withrespect to each other, such that said at least two protrusions on one ofsaid at least two stator components are offset with respect to said atleast two protrusions on an adjacently mounted one of said at least twostator components, such that a portion of each of said windings thatextends beyond a longitudinal end of said protrusions on one of said atleast two stator components fits within a gap between windings mountedon said adjacently mounted stator component.

The technology described herein recognises that the efficiency of anelectric motor falls dramatically when the motor operates away from itsoptimum frequency. Reducing the power supplied to the motor reduces itsoperating frequency but also generally reduces its efficiency. In thisregard in an electric motor where there are windings that generate anelectric field, the motor operates most efficiently if the magneticfield generated within the body that the windings are mounted on issaturated. Supplying a lower current to the windings reduces themagnetic field such that it is no longer a saturated filed and thus,although the rotational speed will drop so also will the efficiency ofoperation of the motor.

The present invention splits the stator of the motor longitudinally intoindividual components such that the windings do not extend along thefull axial length of the stator, only along the length of one of thestator components. In this way driving the windings on only one of thestator components will reduce the power supplied to the motor and thus,the frequency of operation, but will still provide a saturated magneticfield in that part of the stator. However, a problem with such anarrangement is that dividing the windings into several windings in thismanner provides windings that are not as efficient as a single windingwould be, as with windings, the portion of the winding that extendsbeyond the end of the protrusion on which it is mounted does notcontribute much to the magnetic field induced in the protrusion.Dividing the winding into several windings generates more of these endportions. Furthermore, these end portions which extend beyond the end ofthe protrusions make the mounting of the adjacent stator awkward. Inthis regard, when wires are wound around a protrusion, how closely awire can follow the contour of the protrusion is limited by theflexibility of the wire and the angles of the protrusion. The lateralends of the protrusion are quite narrow and, thus the wire needs totravel around two steep angles, which means that the wires do not followthe contours very closely but rather extend out from the protrusions byan amount that is significant enough to affect the compactness of themotor and the efficiency of the magnetic field generation.

The present technique has mitigated this problem by arranging thestators at an offset angle to each other such that the windingsextending out of one stator component fit into the gaps between thewindings sticking out of the adjacent stator components. In this way theend of the windings that extend beyond one stator component may be atleast partially alongside a protrusion in the other stator component.This allows it to contribute to the magnetic field generated in thisprotrusion and also it allows the two stator components to be mountedclose to each other. Thus, the motor can be operated efficiently atfractions of the usual power or rotation frequency at a high efficiency,while not becoming unduly long.

This offset can also in some motor designs reduce torque fluctuationsthat occur when certain parts of the rotor align with the windings onthe stator. In this regard the kick provided in the torque when theactive part of the rotor aligns with an active part of the stator may bereduced by the windings on the stator being located at differentcircumferential positions at different points along its length, makingthe generation of the torque smoother.

Although each stator component may have two or more protrusions in someembodiments each of said at least two stator components comprise atleast twelve protrusions, each comprising a winding.

Providing substantially more than two windings on each of the statorcomponents is a further way of reducing torque fluctuations that arisewhen the active part of the rotor align with the stator windings. Themore stator windings there are then the lower the field produced by eachis, and the corresponding kick provided by each winding to the rotor onalignment is similarly reduced. Thus, the provision of multiple windingsin conjunction with an offset in the windings along the length of thestator can result in reductions in torque ripples.

In some embodiments said rotor comprises a single rotor component.

Although it may be advantageous to provide separate stator components,it may be advantageous for the rotor to be formed of a single componentwhich is mounted within the multiple stator components and driven bythem, the speed and power of the motor depending on the number of statorcomponents that have windings that are powered.

In some embodiments, said rotor comprises a plurality of protrusionsextending out of said outer surface, said plurality of protrusions beingformed of a magnetic material.

Although the rotor may be formed in a number of different ways providedthat it has magnetically active portions that, when close to thewindings of the stator, exert a force on the rotor, a rotor havingprotrusions that are formed of a magnetic material such as iron orsteel, provide a suitable rotor for a switched reluctance motor that canbe driven by appropriate control of the power supplied to the windingsof the stator.

In the case of a switched reluctance motor, in some embodiments a numberof said plurality of protrusions is different to a number of saidprotrusions on each of said at least two stator components.

Where the motor is a switched reluctance motor, although the rotor andstator may have any number of protrusions it may be advantageous if thenumber of protrusions on the rotor is different to that on the statorsuch that they are never all exactly aligned. Once again this willreduce torque fluctuations.

In some embodiments, said plurality of protrusions run parallel to alongitudinal axis of said rotor.

In some embodiments, the protrusions are mounted without an offset suchthat they run parallel along the longitudinal axis of the rotor.Generally when making electric motors there is a prejudice to providethe protrusions at an offset to the longitudinal axis such that oneprotrusion is not exactly aligned with a single winding on the stator atany one time. In this way the torque fluctuations are reduced. However,in some embodiments of the invention it has been found that the motorwill run with reduced torque fluctuations due to other features such asan increased number of windings and some of the windings on the rotorand stator being offset with respect to each other. In such cases it maybe that the provision of protrusions that run parallel to thelongitudinal axis is acceptable and in this way a rotor that is cheaperto manufacture yet provides an acceptable level of torque fluctuationscan be produced.

In some embodiments, said control circuitry is configured toindependently control said power supplied to each of said windings oneach of said at least two stator components in order to control torqueoutput generated by said motor.

Advantageous control over the torque output generated by the motor canbe provided by enabling independent control of the power supplied toeach of the windings on the different stator components. Such atechnique is particularly useful when used in conjunction with thestraight protrusions as the control can be used to compensate for thelack of offset in these protrusions and in effect vary the powersupplied to the windings as the rotor protrusions approach and move awayfrom each of them. In this regard, there has been a technical prejudiceagainst providing individual control signals to control the powersupplied to each of the different windings, particularly where there aremore than three windings that need individual control, as generallypower supplies will not have more than three phases and thus, providingthis additional control is complex. However, it has been recognised thatwith the present technique that provides many windings and thus, areduction in current required in each, then such control may indeed bepossible, as the control components required with the lower current aresignificantly cheaper than those that are required for higher currents.In this way the performance of the motor can be improved particularlywhere rotors with straight protrusions are used.

In some embodiments, said control circuitry is configured to controlsaid power supplied to said windings on each of said at least two statorcomponents such that said power supplied to said windings on one of saidat least two stator components is controlled independently to said powersupplied to windings on said adjacently mounted stator component, andsaid power supplied to each winding on each of said at least two statorcomponents is controlled independently compared to an adjacent winding.

In some cases, rather than providing independent control to eachwinding, the independent control may be limited to windings that are notadjacent to each other and to windings on different stator components.In this way, the number of different control signals is reduced whilestill providing effective control to reduce torque fluctuations.

In some embodiments, said control circuitry comprises rotationalposition detection circuitry for detecting a current position of saidrotor relative to each of said at least two stator components, saidcontrol circuitry being configured to control said power supplied tosaid windings in dependence upon said detected current position.

Where there is independent control to at least some of the windings thena rotational positional detector may be used to determine where therotor is with respect to the stator and thereby provide the appropriatecontrol.

In some embodiments the rotor comprises a plurality of components eachproviding a magnetic path, said plurality of components running parallelto said longitudinal axis. These components may take a number ofdifferent forms, possibly depending on the particular type of motorapparatus. For example, in a “wound rotor” type motor these componentsmay be provided by the windings on the rotor running parallel to itslongitudinal axis. Alternatively where a “squirrel cage” type rotor isprovided, these components may be provided by the electricallyconductive bars which form the squirrel cage. Alternatively where aswitched reluctance type motor is provided, these components may beprovided by structures which enable the electromagnetic response of therotor, such as bars of electromagnetically responsive material. Itshould also be recognised that the magnetic paths may not necessarilycoincide with the components, for example where the arrangement of thecomponents is in the form of fins running along the rotor, where themagnetic paths may be viewed as running along the grooves definedbetween the fins.

In some embodiments said plurality of components are each configured toprovide a single magnetic path running substantially the whole length ofsaid rotor along said longitudinal axis. As such, each component (say awinding, bar or structure as in the examples mentioned above) may forexample itself run substantially the whole length of the rotor (allowingfor perhaps some mountings at either end).

In some embodiments a number of said plurality of components is equal toa number of said plurality of protrusions on said stator. Given thatthere are at least two stator components mounted adjacent to each otheralong said longitudinal central axis and rotationally offset withrespect to each other, this then means that (where these embodiments ofthe rotor have a single rotor structure along the length of the rotor)there are then effectively twice as many stator positions as rotorpositions (in terms of rotational symmetry), giving more precision inthe control that can be asserted over the rotor by the stator.

The present technique is also applicable to inductance motors where therotor has inserts of a highly electrically conductive material in whicha current is induced when moved within an electric and correspondingmagnetic field. These motors tend to be self-aligning. As in inductancemotors the rotor tends to be self aligning with the stator then wherethe number of components is equal to the number of protrusions,offsetting the stator windings with respect to the different statorcomponents will only provide an efficient motor where the rotor isitself formed in two components with the inserts being similarly offsetwith respect to each other. This allows the inserts to be provided in amanner where they run parallel to the longitudinal axis and thus, areeasier to manufacture than those that are offset while still providingthe reduction in torque fluctuations by being offset along their length.In this regard, this will only provide a reduction in torquefluctuations where the motor is run at full power and both statorcomponents are powered. Where only one stator component is powered thenthere will be not be this advantage of the offset along the length ofthe motor. However, as these motors are generally designed with manywindings and inserts this in itself provides a reduction in torquefluctuations.

In some embodiments the rotor comprises at least two rotor componentseach mounted within a corresponding one of said at least two statorcomponents, wherein said plurality of components on each of said atleast two rotor components are each configured to provide a singlemagnetic path running substantially half of the length of said rotoralong said longitudinal axis. Accordingly the components may be formedsplit into two halves along the length of the rotor.

In some embodiments said plurality of components on each of said atleast two rotor components are offset with respect to each other to asame degree as said protrusions on said at least two stator componentsare offset with respect to each other. Accordingly the components may beformed in two halves both split along the length of the rotor andwherein the two halves are also offset (rotationally with respect to thelongitudinal axis) with respect to one another. Offsetting the twohalves with respect to one another provides further as possible rotorpositions (in terms of rotational symmetry), giving more precision inthe control that can be asserted over the rotor by the stator.

A second aspect of the present invention provides method of operating anelectric motor apparatus, said electric motor apparatus comprising: arotor; and a stator formed of at least two stator components, each ofsaid at least two stator components having a substantially hollowcylindrical form; said rotor being mounted within said at least twostator components on a rotational mounting such that said rotor canrotate about a longitudinal central axis with respect to said stator;each of said at least two stator components comprising at least twoprotrusions arranged at different circumferential points on an innersurface of said at least two stator components, each of said at leasttwo protrusions having a winding mounted thereon; and control circuitryto control power supplied to windings on each of said at least twostator components; said method comprising:

supplying power to windings on one of said at least two statorcomponents in response to a request for a low rotational frequency; and

supplying power to windings on more than one of said at least two statorcomponents in response to a request for an increased rotationalfrequency.

A third aspect of the present invention provides an electric motorapparatus comprising:

a rotor; and

a stator formed of at least two stator components, each of said at leasttwo stator components having a substantially hollow cylindrical form;

said rotor being mounted within said at least two stator components on arotational mounting such that said rotor can rotate about a longitudinalcentral axis with respect to said stator;

each of said at least two stator components comprising at least twoprotrusions arranged at different circumferential points on an innersurface of said at least two stator components, each of said at leasttwo protrusions having a winding mounted thereon; and

control means for generating control signals to control power suppliedto said windings on each of said at least two stator components, suchthat power can be controlled to each of said at least two statorcomponents independently of each other; wherein

said at least two stator components are mounted adjacent to each otheralong said longitudinal central axis and rotationally offset withrespect to each other, such that said at least two protrusions on one ofsaid at least two stator components are offset with respect to said atleast two protrusions on an adjacently mounted one of said at least twostator components, such that a portion of each of said windings thatextends beyond a longitudinal end of said protrusions on one of said atleast two stator components fits within a gap between windings mountedon said adjacently mounted stator component.

The above, and other objects, features and advantages of this inventionwill be apparent from the following detailed description of illustrativeembodiments which is to be read in connection with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B and 1C show a data component for an electric motoraccording to an embodiment of the present embodiment;

FIG. 2 shows a rotor for a switched reluctance motor according to anembodiment of the present embodiment;

FIG. 3 shows a rotor for an inductance motor according to an embodimentof the present invention;

FIG. 4 shows a split rotor with two components according to anembodiment of the present invention;

FIG. 5A-C show three different rotor configurations according to threedifferent embodiments of the present invention;

FIG. 6 schematically shows a rotor mounted within a stator of a switchedreluctance motor according to an embodiment of the present invention;

FIG. 7 schematically shows the control of power supplied to windings ona stator of an electric motor according to an embodiment of the presentinvention; and

FIG. 8 shows steps in a method according to an embodiment of the presentinvention.

DESCRIPTION OF EXAMPLE EMBODIMENTS

FIG. 1A shows a stator 10 formed of two components 12 and 14 mountedadjacent to each other. The end section of stator component 12 can beseen and it shows a plurality of protrusions or teeth 20 that extendfrom the inner circumferential surface of the stator and run along thelongitudinal length of the stator. Each of the teeth supports a winding30. The windings 30 are supplied with current and generate a magneticfield within each tooth. The amount of current supplied can be varied,however, where it is just sufficient to generate a saturated magneticfield in a tooth then this provides a particularly efficient motor.

FIG. 1B shows how the windings 30 are mounted along the teeth 20 whichrun along the length of the stator component 12. There are correspondingteeth that run along the length of the stator component 14.

The windings 30 are wound around the teeth and at the end they extendbeyond the end of the teeth by an amount Δt. As will be clear theflexibility of the wire will determine how close to the teeth 20 thewinding 30 can be wound and in general, where the winding passes aroundthe end of a tooth it will extend beyond it. These protrusions Δt impedethe mounting of an adjacent stator.

FIG. 1B also shows power supply circuitry 40 and control circuitry 50which latter controls power supplied from the former to the windings 30on the stator component 12. In this regard, the control circuitry 50 maycontrol the power supply to all of the windings such that all of thewindings in a particular stator component receive the same power at thesame time, or it may provide individual control of the power supplied tothe different windings or it may group the windings together andindividually control the power supplied to these groups of windings. Inany case, the control circuitry 50 allows individual control of thepower that is supplied to the windings on the different statorcomponents 12 and 14 such that one of the stator components can bepowered while the other is not. In this way, the amount of powersupplied to the motor can in this embodiment be reduced by half whilestill providing a saturated magnetic field in the teeth of the statorcomponent to which power is supplied, thereby allowing the motor tooperate efficiently at half the usual power and therefore half thefrequency.

Although, only two components are shown in this example, it should beclear to a skilled person that there could be more than two components,the increasing number of components allowing the motor to operate highefficiency at further power levels and rotational frequencies.

FIG. 1C shows how the problem of the windings 30 extending beyond theend of the teeth 20 by Δt can be addressed to allow the stators to bemounted together in a compact and efficient way. Thus, in FIG. 1C thetwo stator components are rotationally offset with respect to each othersuch that the windings from one extend within the gaps between thewindings on the other. This offset must be sufficient to allow the endsof the windings to overlap and thus, it may be that an appropriateoffset is provided by rotating one of the stator components with respectto the others by half the distance between two adjacent windings. Byoffsetting the stator components in this way not only can the two statorcomponents be mounted in a more compact fashion but the portion of thestator where there are coils which don't overlap with teeth is reduced.In this way the generation of the magnetic field is improved whencompared with the case where the two stator components are not offsetwith respect to each other, so that an extension of one winding does notoverlap with the teeth from another stator.

In the case that they can be mounted to fit exactly within each otherthe distance required between the two stator components is Δt ratherthan at least 2Δt as would be the case if there was no offset.

Offsetting the coils in this way can produce its own problems which canbe addressed in different ways.

In this regard, the fields generated by the two stator components do notline up if the windings are controlled to carry the same currents. Thiscan be a problem particularly in switched reluctance motors and this canbe addressed by providing individual control of the power supplied tothe windings on the different stator components.

Although, this arrangement of the windings on the stator components maygenerate its own requirement for individual control, it can also beadvantageous in that the flux along the stator is not at the same pointin its cycle along the length of the stator and this can be used toreduce torque fluctuations in the motor as it spins the rotor.

As can be seen in FIG. 1A there are a number of protrusions or teeth 20holding a number of windings 30. Although, embodiments of the presentinvention are applicable to stators 10 with two or more such teeth orprotrusions 20, preferred embodiments have many more, for example, 12such protrusions would provide a motor having low torque fluctuations.In this regard, the higher the number of protrusions and windings thelower the torque fluctuations that arise when the active part of therotor is aligned with a particular winding. Furthermore, in order toproduce a motor having a particular power output where there are only afew windings then a large current needs to be supplied to each of thesewindings to provide the required power. However, where there are morewindings the required amount of current is split between the windingsand thus, the actual current supplied to each winding is reduced. Theforce provided by each winding is similarly reduced and fluctuations intorque are also reduced. A drawback of such an arrangement is that itmay require more complex power supply and indeed in some cases control,where the control for each of the windings is done individually.However, as there are lower current requirements for these windings thenit may be that the control circuitry can be formed from switchingcomponents that are more readily available and cheaper making theadditional control requirements cheaper and easier to implement. Suchadditional control can also improve the performance of the motor.

FIGS. 2 to 4 show rotors that are suitable for mounting within thestator shown in FIG. 1. In this regard, FIG. 2 shows a rotor 60 which isformed of a magnetic material in this case silicone steel and which hasprotrusions 62 at different circumferential positions on the outersurface of the rotor. This rotor extends along the full length of thestator 10 within both of the stator components 12 and 14. The number ofprotrusions 62 it has are not equal to the number of teeth that holdwindings in the stator of FIG. 1 such that when one of the teeth of therotor is aligned with a winding in the stator the adjacent ones are notaligned.

As can be seen from this embodiment the protrusions 62 extend in astraight line that is parallel to the longitudinal axis xx of the rotor.This enables the rotor 60 to be more easily built than were theprotrusions arranged at an offset to the longitudinal axis as is commonin the prior art. Allowing the protrusions to run in a straight linedoes mean that they are aligned with some of the windings for a longerlength of the stator which makes the generation of the torque lessuniform. However, this is acceptable in a design where there are manyprotrusions and windings such that the force from each winding isreduced compared to a design with fewer windings and protrusions thatgenerate the same resultant force.

Furthermore, where more than one stator component is powered up then thewindings in the different stator components are offset with respect toeach other along the stator length and this in itself reduces torquefluctuations. In such a case, the powering of the windings must becontrolled so that the appropriate power is supplied at the appropriatetime to drive the rotor in the required direction. This will beexplained in greater detail with respect to FIG. 7.

FIG. 3 shows an alternative rotor 70 which can be mounted within thestator 10 of FIG. 1. This rotor is for use in an inductance type motorand comprises inserts 72 that are made of a highly conductive material.When the rotor 70 is mounted within the stator 10 current flowing in thewindings generates an electric and magnetic field which induces acurrent within the insert 72, which in turn generates an electric andmagnetic field which provides a force that acts on rotor 70 and causesit to rotate. In this embodiment, the number of inserts is designed tobe equal to the number of teeth on the stator 10 and the device is selfaligning. Rotor 70 may not rotate very efficiently within the stator 10of FIG. 1 owing to the offset of the stator components when they areboth powered.

A preferred form of the rotor 70 is shown in FIG. 4. In this embodimentthe inserts 72 are offset which respect to each other at a point alongthe length of the rotor, in a way that corresponds to the way that thewindings in the stator components 12 and 14 10 of FIG. 1A are offsetwith respect to each other. This allows the self aligning of the rotorto be consistent along the length of the stator component and leads toan efficient motor. Although, FIG. 4 and FIG. 1A both show twocomponents for rotor and stator respectively it should be understood bya skilled person that there could be many more, with each componenthaving an offset that allows the windings on one stator component toslot within the gaps between the winding on one or more adjacent statorcomponents. The inserts on the rotor should be offset in a correspondingmanner to allow them to match the offset of the windings of the stator.

FIGS. 5A-5C schematically show three different rotor configurationsaccording to three different embodiments. The longitudinal darker barsin each of FIGS. 5A-C represent components which each provide aelectrical path, whilst the paler sections between the darker barsprovide magnetic paths. The particular type of these components willvary depending on the particular type of motor apparatus. For example,where the motor has a “wound rotor” these components represent thewindings on the rotor running parallel to its longitudinal axis.Alternatively where a “squirrel cage” type rotor is provided, thesecomponents represent the electrically conductive bars which form thesquirrel cage. Alternatively where a switched reluctance type motor isprovided, these components represent the structures which enable theelectromagnetic response of the rotor, such as bars ofelectromagnetically responsive material.

In the example of FIG. 5A these components each run substantially thewhole length of the rotor along its longitudinal axis (other than themountings (darker shaded ring) at either end). The magnetic pathsprovided by these components thus also run substantially the wholelength of the rotor along its longitudinal axis. The number of thesecomponents may be set to be equal to the number of protrusions on saidstator. This style of rotor has a manufacturing simplicity and can alsoallow for torque ripple reduction due to the apparent doubling of thepositions of the stator teeth that results in a configuration when thenumber of components matches the number of stator teeth (due to therotational offset between the (at least) two stator components).

In the example of FIG. 5B there are shown to be two halves to the rotorand the components on each of the two halves provide a single magneticpath running substantially half of the length of said rotor along saidlongitudinal axis. The two halves are divided by another mountings(darker shaded ring) shown at the midpoint of the rotor). Like the FIG.5A configuration this can also support torque ripple reduction, but theFIG. 5B configuration also allows entirely half the motor to be shutdown completely. This saves magnetic core losses in half the rotor andstator cores when the motor is used in low power output situations.

In the example of FIG. 5C it can be seen that, like in FIG. 5B, thereare two split halves to the rotor, but in addition these two halves arerotationally offset with respect to one another. Although this then doesnot support the torque ripple reduction effect possible with the FIGS.5A and 5B configurations, like the FIG. 5B configuration this alsoallows entirely half the motor to be shut down completely. Moreover,this configuration can simplify the electronics since now the two halvesof the motor look identical in electrical timing and so half as manycalculations and control circuitry is necessary.

FIG. 6 shows a cross section through a motor with a rotor 80 which is arotor similar to that shown in FIG. 2 but with only two protrusionsmounted within a stator 15. In this case the stator 15 has three teeth17 each comprising windings and the rotor 80 has two teeth 82. As can beseen when one of the teeth 82 is aligned with one of the teeth 17 of thestator then the other tooth is directly between the other twoprotrusions. This means that the force generated from the winding is ata maximum on one of the teeth of the rotor and a minimum on the other.Such an arrangement allows the torque supplied to the rotor to be fairlyconstant as the rotor 80 revolves. Clearly in many embodiments therewill be many more teeth on both the stator and the rotor which will leadto even fewer fluctuations in torque.

FIG. 7 shows a control system for a switched reluctance motor such asthat shown in FIG. 6 but with more teeth on both the rotor and thestator. In this embodiment, the windings 30 on stator 10 are poweredindividually by a power supply 40 which has a multi-bit control signalsent from control circuitry 50. There is also a position detector 55which determines a current position of a rotor. This may be doneoptically using a mark on the rotor which is detected as it passes theoptical sensor or in some other known way. The position detectordetermines the rate of rotation of the rotor and the current position ofthe protrusions on the rotor and this information is used to control thecurrent supplied to each of the windings such that they are at apreferred level compared to the position of the protrusions on the rotorand act to efficiently pull the rotor around. In this way, one canensure that the magnetic field pulling one portion of the rotor does notact against the magnetic field pulling another portion of the rotor.

In this embodiment the protrusions 62 on the rotor are aligned with thelongitudinal axis of the rotor, which means that when a protrusion isaligned with a winding, it is aligned along the entire length of atleast one stator component. This can lead to fluctuations in the torquegenerated. This can be addressed by individual control of the currentsupplied to the windings, such control acting to control these torquefluctuations.

FIG. 8 shows a flow diagram illustrating steps in a method according toan embodiment of the present invention. The initial step is to determinewhether or not the electric motor is required operate at a high power.If it is then power is supplied to all the windings in all of the statorcomponents. If it is not then power is supplied to the windings in asub-set of the stator components. In the case of a switched reluctancemotor then the power is supplied in pulses whereas in an induction motorthe power is generally supplied as a sinusoidal wave.

In the case of an induction motor the number of stator components thatare powered determines the power of operation of the motor. The motoroperating at high efficiency as the windings that are powered are fullypowered.

In the case of a switched reluctance motor then there is an additionalcontrol of the power that is sent to the individual windings on thestator teeth of each stator component. Thus, for each stator toothwinding the relative position of the rotor is determined with respect tothe stator and where the rotor tooth is approaching the stator tooth apulse of power is supplied to the winding on this stator tooth toattract the rotor.

It is then determined if there is a new control signal. If not this loopis performed again whereas if there is a new control signal it isdetermined if the motor should still be operating at high power or if itshould be operating at lower power. If so the power that is supplied tothe different stator components is changed in dependence on the answerto this question. In this way, in a switched reluctance motor thepulsing of the power is controlled to the individual windings whichallows for it to operate efficiency and ripples in the torque to bereduced even where the teeth on the rotor are straight rather than beingoffset to the longitudinal axis in the form of a squirrel cage.

It should be noted although an example was given here of individualcontrol of the windings they may in fact be controlled in sets ofwindings. This may be appropriate where for example, the number of teethon the rotor and stator are such that alternate windings are at a samerelative position to the rotor teeth and should be controlled in asimilar manner.

Although illustrative embodiments have been described in detail hereinwith reference to the accompanying drawings, it is to be understood thatthe claims are not limited to those precise embodiments, and thatvarious changes and modifications can be effected therein by one skilledin the art without departing from the scope and spirit of the appendedclaims. For example, various combinations of the features of thefollowing dependent claims could be made with the features of theindependent claims.

1. An electric motor stator comprising: first and second stator components arranged adjacent to each other along a longitudinal central axis of the stator, each stator component having a substantially hollow cylindrical form for receiving a rotor such that said rotor can rotate about the longitudinal central axis of the stator, the first and second stator components each comprising at least two protrusions arranged at different circumferential points on an inner surface of the respective stator component, wherein the first and second stator components are arranged such that the protrusions on the first stator component are rotationally offset with respect to the protrusions on the second stator component, such that when each of the protrusions have a winding mounted thereon, a portion of a winding on the first stator component extends into a gap between windings on the second stator component.
 2. An electric motor stator according to claim 1, comprising one or more windings mounted to one or more of said protrusions.
 3. An electric motor stator according to claim 2, wherein each of said first and second stator components comprises at least twelve said protrusions, each comprising a said winding.
 4. An electric motor stator according to claim 1, wherein each of said windings of each of said at least two stator components are independently controllable.
 5. An electric motor stator according to claim 4, wherein said windings on one of said at least two stator components are powerable independently to said windings on said adjacently mounted stator component.
 6. An electric motor comprising: a rotor; and the electric motor stator according to claim 1; said rotor being mounted within said at least two stator components on a rotational mounting such that said rotor can rotate about a longitudinal central axis with respect to said stator.
 7. An electric motor controller for controlling an electric motor according to claim 6, comprising: control circuitry to generate control signals to control power supplied to said windings on each of said at least two stator components, such that power can be controlled to each of said at least two stator components independently of each other.
 8. An electric motor controller according to claim 7, wherein said control circuitry is capable of independently controlling said power supplied to each of said windings on each of said at least two stator components in order to control output torque generated by said motor.
 9. An electric motor controller according to claim 7, wherein said control circuitry is capable of controlling said power supplied to said windings on each of said at least two stator components such that said power supplied to said windings on one of said at least two stator components is controlled independently to said power supplied to windings on said adjacently mounted stator component, and said power supplied to each winding on each of said at least two stator components is controlled independently compared to an adjacent winding.
 10. An electric motor controller according to claim 8, wherein said control circuitry comprises rotational position detection circuitry for detecting a current position of said rotor relative to each of said at least two stator components, said control circuitry being configured to control said power supplied to said windings in dependence upon said detected current position. 